2. An Organism’s Responses to changes in their Environment

3. Stimulus Any change in an organism’s internal or external environment that causes the organism to react to the change. Stimuli – plural (more than one stimulus)

4. Response The reaction of an organism to a stimulus.

5. Examples of stimuli and their responses: The dog is hungry, so it eats some food from the bowl A rabbit sees a wolf, so it runs away to safety

6. Plants also respond to their environment. When plants grow towards the sun, we call this reaction phototropism

7. Plants respond to their environment in other ways too. Plants roots grow down towards the water due to the effect of gravity

8. Two Types of Stimuli Internal External

9. Internal stimulus A stimulus that comes from inside the organism. When the body becomes too warm due to exercise, the skin responds by sweating. The sweat evaporates and cools the body temperature down.

10. Internal Stimuli & Responses Think of all the changing conditions that an organism might need to respond to. Here’s just a few samples: And here are some specific responses to these internal stimuli. Forget shivering… Where’s my hoodie?!?

11. When the finger senses that something is dangerously hot, it pulls away. External stimulus A stimulus that comes from outside of the organism.

12. External Stimuli & Responses External stimuli can be more complicated. Let’s see how. EXTERNAL STIMULUSRESPONSE Bright sunlightSquinting (ok, this one is pretty simple) Shorter days, colder temperatures Trees drop their leaves in autumn Increasingly hot weather Dogs shed their thick winter coats in late spring Sunlight, motion of the Sun.Plants grow toward the Sun’s position. Difficult survival conditions coming up Hibernation (chipmunks sleep thru cold months) Migration (geese fly to warm weather & easy food) Dormancy (grass turns brown in winter instead of trying to grow & stay green) There really are countless more examples, from as simple as an itch on your arm to the complicated conditions that cause plants to realize winter is over and spring back into life.

13. When there is not enough light to see, the pupil of the eye enlarges to allow as much light in as possible so you can see. When the light is too bright, the pupil size becomes smaller to prevent too much light from going in and temporarily blinding you. pupil Internal or External Stimulus? External

14. When pollen enters your nose, the body responds by sneezing. External Internal or External Stimulus?

15. When growth hormones help you add more muscle & bone tissue during growth spurts Internal Internal or External Stimulus?

16. The End!!